For the purpose of burglar-proof, ventilation or natural light, a first door and a second door are mounted on a door frame, and a first lock and a second lock are respectively mounted on the first door and the second door. While intending to open the first and second doors, the first door has to be opened firstly via the first lock on the first door, and thus the second door can be opened via the second lock on the second door, but it is not convenient for use. In addition, more space is required for the first and second doors when they open in different directions.